Ceramics possess attractive mechanical and thermal properties. A major limitation to their use is a lack of strength and toughness. Unlike most polymers and metals, ceramics generally are unable to absorb damage without catastrophic failure. Much attention has been given to developing strength and toughness in ceramics.
One route to improved ceramic strength is based on the transformation of zirconia from the tetragonal to the monoclinic phase with resulting increase in volume. This phenomenon is reported in both patent and scientific literature (Lange, U.S. Pat. No. 4,316,964; J. Materials Science 19, 2233 (1984). Zirconia is also known in the art (J. American Ceramic Society, 67, N.3, 164 (1984); J. of Materials Science Letter 4, 1067 (1985) to inhibit grain growth in ceramics. High strength ceramic fibers which contain at least 60% alumina are taught in Seufert, U.S. Pat. No. 3,808,015. An object of this invention is to provide alumina fibers of greater strength and strength retention after high temperature exposure.